The jury is hearing the opening statements and will consider the first evidence about the massacre Cruz perpetrated that left 17 people dead at the Stoneman Douglas High School in Parkland on February 18, 2018.
Cruz, 23, pleaded guilty last October to 17 counts of first-degree murder and 17 counts of attempted first-degree murder during his rampage at the high school.
Under Florida state law, the jury must reach a unanimous decision to put forth a death sentence.
Legal experts have said they expect the prosecutor in the case to emphasize Cruz's brutality as he systematically killed pupils and staff and sometimes walked back to the wounded victims to shoot them again to ensure they were dead.
The trial is expected to last at least several months, according to the experts.